US1261
APPLICATION INFORMATION
selected heat sink be chosen such that during maxi-
mum continuous load operation the junction tempera-
ture is kept below this number. Two examples are given
which shows the steps in selecting the proper regulator
heat sink for driving the Pentium II processor GTL+ ter-
mination resistors and the Clock IC using 1261 in TO220
and TO-263 packages.
Introduction
The US1261 is a dual fixed output Low Dropout (LDO)
regulator available in a 5 pin TO-220 or TO-263 pack-
ages. This voltage regulator is designed specifically for
PentiumII processor applications requiring 2.5V and 1.5
V supplies, eliminating the need for a second regu-
lator resulting in lower overall system cost. The
US1261 is designed to take advantage of 5V supply to
provide the drive for the pass transistor, allowing 2.5V
supply to be generated from 3.3V input.This feature im-
proves the power dissipation of the 2.5V regulator sub-
stantially allowing a smaller heat sink to be used for the
application. Compared to the US1260 dual adjustable
regulator, the US1261 includes the resistor dividers that
are otherwise needed with the US1260, eliminating 4
external components and their tolerances, resulting in a
more accurate initial accuracy for each output voltage.
Other features of the device include; fast response to
sudden load current changes, such as GTL+ termina-
tion application and thermal shutdown protection to pro-
tect the device if an overload condition occurs.
Example # 1
Assuming the following specifications :
VIN = 3.3V
VOUT 2 = 1.5 V
VOUT 1 = 2.5 V
IOUT 2 MAX = 5.4A
IOUT 1 MAX = 0.4 A
TA = 35°C
The steps for selecting a proper heat sink to keep the
junction temperature below 135°C is given as :
1) Calculate the maximum power dissipation using :
PD = IOUT1 ´ VIN - VOUT1 + IOUT2 ´ VIN - VOUT2
PD = 0.4 ´ 3.3 - 2.5 + 5.4 ´ 3.3 - 1.5 = 10 W
(
)
(
)
2) Select a package from the datasheet and record its
junction to case (or Tab) thermal resistance.
Selecting TO220 package gives us :
Stability
The US1261 requires the use of an output capacitor as
part of the frequency compensation in order to make the
regulator stable. Typical designs for the microproces-
sor applications use standard electrolytic capacitors with
typical ESR in the range of 50 to 100 mW and the output
capacitance of 500 to 1000uF. Fortunately as the ca-
pacitance increases, the ESR decreases resulting in a
fixed RC time constant. The US1261 takes advantage of
this phenomena in making the overall regulator loop
stable. For most applications a minimum of 100uF alu-
minum electrolytic capacitor with the maximum ESR of
0.3W such as Sanyo, MVGX series ,Panasonic FA se-
ries as well as the Nichicon PL series insures both sta-
bility and good transient response. The US1261 also
requires a 1 uF ceramic capacitor connected from Vin
to Vctrl and a 10W, 0.1W resistor in series with Vctrl pin
in order to further insure stability.
qJC =2.7°C/W
3) Assuming that the heat sink is Black Anodized, cal-
culate the maximum Heat sink temperature allowed :
Assume , qCS = 0.05 °C/W (Heat sink to Case thermal
resistance for Black Anodized)
TS = TJ - PD ´ qJC + qCS
TS = 135 - 10 ´ 2.7 + 0.05 = 107.4 °C
(
)
4) With the maximum heat sink temperature calculated
in the previous step, the Heat Sink to Air thermal resis-
tance qSA is calculated as follows :
DT = TS - TA = 107.4 - 35 = 72.4 °C
DT
qSA =
PD
Thermal Design
72.4
qSA =
= 7.24 °C / W
The US1261 incorporates an internal thermal shutdown
that protects the device when the junction temperature
exceeds the maximum allowable junction temperature.
Although this device can operate with junction tempera-
tures in the range of 150°C ,it is recommended that the
10
5) Next , a heat sink with lower qSA than the one calcu-
lated in step 4 must be selected. One way to do this is
to simply look at the graphs of the “Heat Sink Temp
Rise Above the Ambient” vs. the “Power Dissipation” and
Rev. 1.7
3/22/99
3-4
UNISEM [ UNISEM ]
